My guess is that your resonator isn't on your Jeep and your semi-informed dealer is confusing that with the cats.
The resonator sits horizontal to the ground in the single exhaust line, almost directly under the driver. It looks like a bullet muffler. It then feeds downstream, over the axles and into the muffler. The muffler sits directly under your rear cargo area and looks like a trashcan mounted sideways (it's about the size of a trashcan).
The cats are upstream of the resonator, one on each side. off the exhaust manifolds as shown in the diagram bfschor posted. They're a biatch to get to if you have to service them. Jeeps have two cats, one per side. Many of today's vehicles have four (two per side), upstream and downstream cats.
The picture below shows where I straight piped in place of my resonator. If you don't see what looks like a muffler where I've circled in red (your resonator) then it's been removed and the dealer is likely mistaking this for a cat.